Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of 13x9-inch pan with shortening or cooking spray.
In large bowl, beat cake mix, water, oil, eggs and mashed bananas with electric mixer on low speed 30 seconds; beat on medium speed 2 minutes, scraping bowl occasionally. Pour into pan.
Bake 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
Poke top of cake every 1/2 inch with long-tined fork, wiping fork occasionally to reduce sticking. In large bowl, stir together condensed milk, coconut milk and whipping cream. Carefully pour evenly over top of cake. Cover; refrigerate at least 2 hours or overnight until mixture is absorbed into cake.
Spread frosting over cake. Garnish each serving with banana slices and/or toasted coconut, if desired. Store covered in refrigerator.
High Altitude (3500-6500 ft): Bake 35 to 40 minutes.